Benchmark Sampling Systems for Material Balance: Part 2

Promi-Mex’s ally Francis Pitard, President of Francis Pitard Sampling Consultants, and an expert and author on the Theory of Sampling explained to MBN the foundation and basics of this theory. Pitard shared the foundations of the theory and its importance in the first part of this article.

In regards to the Theory of Sampling, cutters play an important role in the process of sampling. A correct way for the cutters to be positioned is where both cutter edges are straight and perpendicular to the cutter trajectory. An incorrect way, and where many manufacturers fail to succeed is to position the trailing edge perfectly superposable to the leading edge, by a translation for straight path cutters, or by a rotation for circular path cutters.

The thickness of the cutter edges and shapes of the cutter edges plays a critical role in a correct sampling, explained Pitard. However, one must not lean to adjustable cutter edges, as they may ruin the correctness of the sampling systems. “The leading edge of a rotating cutter may always be dirty, while the trailing edge stays clean. Also, adjustable plates are rarely kept perfectly symmetrical,” said Pitard.

Pitard shared the primary rules for benchmark sampling systems for material balance. The first rule is that reliable sampling for material balance cannot be performed accurately on coarse material. “The most vulnerable area is sampling the feed going to the plant.” The second rule is to know the heterogeneity of the constituent of interest. Mineralogical studies are essential at the beginning of any new project and in the past, heterogeneity tests used to be recommended. “Today, a better way is to make sure geologists collect the relevant information about the size, frequency and distribution of the coarse particles of the constituent of interest on the exploration log. Then, and only then, sampling, subsampling, and analytical aliquot mass can be optimized,” said Pitard.

The third rule is the selection of equi-probabilistic sampling systems for the selection of mechanically correct sampling systems. The inescapable and most logical and economical sequence bases on the selection of a reliable sampling expert, establishing clear objectives with the client, designing conceptually the most reliable sampling system, selecting the most appropriate manufacturer, and contacting engineering firms and give them appropriate directives.

When looking for a primary sampler for a flotation plant, Pitard recommends to first look for a lateral view with the right length and depth to avoid the contamination of the sampling. The front view of the sampler should have at least 15cm away from the stream on each side, with a stream discharge of 230cm. “An inspection of each parking side should be carried out to additionally ensure 30cm separation between the parking place.”

For a launder between primary and secondary samplers for a flotation plant, one must look for inspection doors to see inside, have automatic sprayers to avoid sickness and calculate a diameter for this sampler to be able to take at least 7 to 10 cuts from each primary cut. For secondary and tertiary samplers for a flotation plant, the best option is rotating vezin samplers.

As for a primary sampler for solids, one must look for equipment where material cannot be contaminated. “It is nearly impossible to extract, crush, grind, pulverize, or screen materials without introducing some contamination from the equipment, or from the surrounding equipment. Thus, the best preventive action is to make sure that important contaminants are excluded, or at least present in a negligible amount,” explained Pitard. Which contaminants can be tolerated and which cannot depends on objectives given to the sample, therefore, it is not enough to simply list general conditions to minimize contamination. In addition, the operator must be aware of the purpose of sampling to prevent the use of equipment incompatible with the sample.

To minimize contamination by dust, Pitard recommends to first, reduce free falls of the material to be sampled to a minimum by suppressing long chutes from conveyor belts, feed riffle splitters slowly and for small lots, vibratory feeders are recommended.  Also, enclose all sources of dust inside hoods with slight negative air pressure is recommended, since  much ventilation is always detrimental. Finally, protect cutters on idle position with caps to prevent the collection of dust at all times.

Contamination can also be present by material in the sampling circuit, the first way is when the same material is submitted to the sampling circuit at regular intervals. To avoid this, Pitard said to make sure between each operation the content of the constituent of interest does not change very much. “Quick cleaning between each operation is necessary,” stressed Pitard, “while thorough and complete cleaning may not be a must.”

The second case of contamination is when different materials are submitted to the sampling circuit. To minimize this problem, “it is a must to process feed, concentrates, and tailings with different equipment installed in different rooms,” said Pitard. If this is not possible, the sampling circuit must be thoroughly cleaned between each operation. The circuit can be fed with material similar is composition to the next real material to be sampled and prepared.

According to Pitard, contamination by abrasion is also likely to occur. “Crushing, grinding, pulverizing, screening, and to a lesser extent all handling operations performed on abrasive materials are likely to introduce in a sample small amount of material abraded from the equipment.”

And as such, contamination by corrosion of the sampling and preparation equipment is likely to take place with aggressive materials such as wet materials developing acid reactions. In a large majority of cases the solution is to build sampling equipment with stainless steel of excellent quality.